In fact, Blended Winglets can, and should be, applied to all 25,000 commercial aircraft in the market, believes Clark
(Joe Clark, CEO, Aviation Partners Boeing).
From 'Airline Fleet and Asset Management'
Doesn't quite square with the theory that early 738's can't be retrofitted.
They can also fit 73 Classics, 757's, 767's and 747's.
My theory is that Ryanair are returning aircraft to the lessors before the maintainance costs get too high.